Metal V-belts such as this have been known for some time, and are disclosed, for example, in Japanese Utility Model Laid-Open No. 62-131143, Japanese Patent Laid-Open No. 2-225840, and Japanese Patent Laid-Open No. 7-12177. These metal V-belts used in the past comprise an endless belt-form metal ring member and numerous metal element members (also called metal links) supported along this metal ring member, and are looped between a drive pulley and a driven pulley so as to transmit power. These two pulleys are designed so that the width of the V-groove thereof can be varied, and are designed so that the loop radius of the V-belt can be varied and the gear ratio continuously varied by varying this V-groove width.
When power is transmitted between the two pulleys by a metal V-belt such as this, the metal element members are pushed upon while the power is transmitted, so that the power is transmitted by the compression force acting on the metal element members. At this time a tensile force acts on the metal ring member that ties the numerous metal element members together in a ring, and flexural stress that varies with the rotation also acts on this metal ring member while the V-belt is looped and rotating between the two pulleys, with the tension and flexion repeatedly acting according to the rotational period of the metal V-belt between the two pulleys.
It is therefore necessary to take into account the tensile and flexural stress that thus repeatedly act on the metal ring member, and to optimize the material, shape, and so on thereof so that the strength and service life will be satisfactory with respect to this repeated stress. The metal ring member generally comprises a plurality of superposed, thin, endless belt-form metal ring sheets, and since power is transmitted in a state in which the innermost metal ring sheet is in contact with the metal element members, it is necessary at the design stage thereof to take into account the coefficient of friction between the metal ring sheets themselves and the coefficient of friction between the metal ring sheets and the metal elements.
Flexural stress can be unequivocally determined by the loop radius of the two pulleys, but tensile stress varies with each metal ring sheet depending on the above-mentioned coefficient of friction. In general, the coefficient of friction between the metal ring sheets (ring-ring coefficient of friction) is smaller than the coefficient of friction between the metal ring sheets and the (saddle surfaces of the) metal elements (ring-element coefficient of friction), so the change in tension is greatest at the innermost metal ring sheet, and when all of the metal ring sheets have the same thickness, the change in tensile stress is greatest for the innermost metal ring sheet. In the past, the metal rings comprised a plurality of metal ring sheets of the same material and the same thickness superposed in the radial direction, so the stress conditions were most severe at the innermost metal ring sheet for the above reason, and the innermost metal ring sheet was the most problematic in terms of strength and service life.